Edward Snowden has dismissed as “false” the New Zealand PM’s claims of no mass surveillance in the country. The whistleblower says he regularly “came across communications of New Zealanders,” when he worked as an NSA analyst.

New Zealand’s Government Communications Security Bureau (GCSB) has performed mass spying, despite denials by the nation’s prime minister, John Key, the former NSA contractor says in his article, issued on Monday by the Intercept.

“Let me be clear: any statement that mass surveillance is not performed in New Zealand, or that the internet communications are not comprehensively intercepted and monitored, or that this is not intentionally and actively abetted by the GCSB, is categorically false. If you live in New Zealand, you are being watched,” Snowden wrote.

The revelation comes shortly after US journalist Glenn Greenwald claimed there was proof of mass snooping being carried out in New Zealand among the NSA files, Snowden leaked.

That was met with a harsh response from Key, who vigorously denied claims by Greenwald and described the journalist as “just another guy who wants to create a conspiracy theory.”

Key argues New Zealand looked into the possibility of what he described as “mass protection,” but has never eventually chosen that option.

Refuting the PM’s assertion, Snowden recalls his own experience as an analyst for US intelligence.

“At the NSA I routinely came across the communications of New Zealanders in my work with a mass surveillance tool we share with GCSB, called ‘XKeyscore’,” he wrote.

Classified documentation of XKeyscore was made available online in July, 2013.

Snowden now draws attention to a ‘Five Eyes Defeat’ filter in the system, which is a checkbox allowing analysts to exclude from their search results coming from the Five Eyes nations – the US, UK, Canada, Australia, and New Zealand.

“Ask yourself: why do analysts have a checkbox on a top secret system that hides the results of mass surveillance in New Zealand if there is no mass surveillance in New Zealand?” Snowden says.

The whistleblower claims New Zealand’s intelligence operatives are not passive consumers of the information they get through XKeystore, “but also actively and directly develop mass surveillance algorithms for it.”

“The claim that it never went ahead, and that New Zealand merely ‘looked at’, but never participated in, the Five Eyes’ system of mass surveillance is false, and the GCSB’s past and continuing involvement with XKeyscore is irrefutable,” Snowden says.

Key earlier said that all of the files Snowden leaked had been stolen before New Zealand made its final decision on surveillance. He also promised to declassify some documents proving his point.